Helical racemate architecture based on osmium(II)-polypyridyl complexes: Synthesis and structural characterisation

Article abstract of DOI:10.1016/j.jorganchem.2005.03.051

New polypyridyl osmium(II) complexes [Os(κ³-tptz)(EPh ₃)₂Cl]BF₄ (E = P, 1; As, 2) with group 15 donor ligands are reported. Structural studies on the representative complex [Os(κ³-tptz)(PPh₃

Full text of DOI:10.1016/j.jorganchem.2005.03.051

Journal of Organometallic Chemistry 690 (2005) 3105–3110
www.elsevier.com/locate/jorganchem
Helical racemate architecture based
on osmium(II)-polypyridyl complexes: Synthesis
and structural characterisation
*
Sanjay Kumar Singh, Sanjeev Sharma, Manish Chandra, Daya Shankar Pandey
Department of Chemistry, Awadhesh Pratap Singh University, Rewa-486 003 (M.P.), India
Received 11 December 2004; revised 10 March 2005; accepted 14 March 2005
Available online 23 May 2005
Abstract
New polypyridyl osmium(II) complexes [Os(j3-tptz)(EPh3)2Cl]BF4 (E = P, 1; As, 2) with group 15 donor ligands are reported.
Structural studies on the representative complex [Os(j3-tptz)(PPh3)2Cl]BF4 revealed formation of helical racemates with sidewise
stacking of right and left-handed anti-parallel helical strands. Salient structural features and DNA binding studies along with binding
constant [6.6 · 103 Mꢀ1] and site size [0.12] of the complex 1 with calf thymus (ct) DNA by absorption spectroscopy are described.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Osmium(II)-polypyridyl complexes; Group 15 donor ligands; X-ray; Weak interactions; DNA binding constant; Helical racemates
1. Introduction
ligands [5a]. We have reported a new series of complexes
[Ru(j3-L)(EPh3)2Cl]BF4 (E = P, As and L = tpy, tptz)
containing both group 15 and polypyridyl ligands and
have shown that these could be employed as precursor
in the synthesis of other ruthenium complexes and that
they behave as metallo-intercalators [5b,5c]. To examine
the effect of the metal core and uncoordinated sites on
the reactivity and DNA binding activity of the com-
plexes, we have prepared analogous osmium complexes
[Os(j3-tptz)(EPh3)2Cl]BF4 (E = P, As). Due to the pres-
ence of uncoordinated nitrogen donor sites on tptz li-
gand in these complexes, they may behave as potential
metallo-ligands. Absorption titration studies on the os-
mium complexes with ct DNA indicated a moderate
interaction with calf thymus DNA presumably in an
intercalative manner between the base pairs of the
DNA helix. The complexes under investigation present
the first example of osmium complexes containing both
group15 and polypyridyl ligands [6]. In this short commu-
nication, we present preliminary results on the synthetic,
spectral, structural including weak interactions studies
and DNA binding behavior of the osmium complexes
The investigations of polypyridyl transition metal
complexes are of particular interest because of their po-
tential use in many areas [1,2]. Self-assembled molecular
helices are found in many biologically important macro-
molecules [3]. Formation of the helicates is governed by
choice of the ligand, metal core and sometimes by reac-
tion conditions [4]. A linear combination of two com-
plex units lead to helicates but the binding to the
metal centers in these units, determines twisting or side-
wise stacking of the units. Vander Waals interaction
play a vital role in the stabilization of such huge archi-
tectures. Further, it has been shown that transition me-
tal complexes containing planar polypyridyl ligands are
of significant importance in DNA binding studies
[5b,5c]. Recently, we have been interested in the synthe-
sis and characterization of metallo-ligands/synthons
based on organometallic systems containing polypyridyl
*
Corresponding author. Tel./fax: +91 7662 230684.
E-mail address: dsprewa@yahoo.com (D.S. Pandey).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.03.051

3106
S.K. Singh et al. / Journal of Organometallic Chemistry 690 (2005) 3105–3110
+
N
N
N
MeOH/Reflux
N
(NH ) [OsCl ] +
+
EPh
tptz
4 2
6
3
N
N
Os
Ph E
EPh
3
3
EPh = PPh ;
1
2
3
3
Cl
AsPh ;
3
Scheme 1.
Table 1
1H NMR data for complexes 1 and 2 relative to free tptz ligand
Free tptz ligand
Complex 1
Complex 2
2
0
00
)
d 8.98 (d, 3H1;1 ;1
d 9.18 (d, 1H1)
d 8.19 (t, 1H2)
d 8.80 (t, 1H3)
d 9.08 (d, 1H4)
d 9.21 (d, 1H1)
d 8.27 (t, 1H2)
d 8.96 (t, 1H3)
d 9.04 (d, 1H4)
3
4
1
0
00
)
d 7.58 (t, 3H2;2 ;2
d 8.01 (t, 3H3;3 ;3
N
0
00
)
0
00
)
d 8.91 (d, 3H4;4 ;4
N
N
0
00
0
00
d 8.66 (d, 2H4 ;4
)
d 8.72 (d, 2H4 ;4
)
''
4
'
'
4
''
3
0
00
0
00
'
3
d 8.09 (t, 2H3 ;3
)
d 8.12 (t, 2H3 ;3
)
N
0
00
0
00
d 7.86 (t, 2H2 ;2
)
d 7.59 (t, 2H2 ;2
)
N
N
'
''
2
2
''
0
00
0
00
1
d 9.59 (d, 2H1 ;1
)
d 9.62 (d, 2H1 ;1
)
1
M
with calf thymus DNA along with the binding constant
ꢀKbꢁ and site size value ꢀsꢁ.
of EPh3 (E = P; 1 and As; 2) under refluxing conditions,
afforded a new series of cationic complexes [Os(j3-tptz)-
(EPh3)2Cl]+ in good yields (Scheme 1). The complexes
were isolated as their tetrafluoroborate salts [7].
2. Results and discussion
The cationic complexes [Os(j3-tptz)(EPh3)2Cl]+ pos-
sessing j3 bonded tptz, two tertiary phosphines or ar-
sines and a labile chloro group have the potential to
exhibit rich chemistry. Further, due to the presence of
Reactions of ammonium hexachloro osmate(II) with
2,4,6-tris(2-pyridyl)1,3,5-triazine (tptz) in the presence
Fig. 1. X-ray crystal view for the complex 1.
Fig. 2. Helical racemate architecture of complex 1.

S.K. Singh et al. / Journal of Organometallic Chemistry 690 (2005) 3105–3110
3107
uncoordinated donor sites on tptz, these have the poten-
tial to behave as metallo-ligands and could find applica-
tion in the synthesis of homo/hetero bi/poly nuclear
systems. Like, analogous ruthenium complexes it re-
acted with NaCN and sodium diethyldithiocarbamate
to give analogous substitution products as [Ru(j3-
tptz)(PPh3)(CN)2] and [Ru(j3-tptz)(PPh3)(dtc)]+, respec-
tively [5b]. The structural identity of complexes 1 and 2
were established by a set of well-resolved signals in the
1H NMR spectra of the complexes in CDCl3 (Table
1). All the protons of the coordinated tptz ligands
are shifted downfield with respect to that of the free
ligand in both the complexes. The spectra were fully as-
signed with the help of 1H–1H COSY and data are
resembled with the proposed structure. 31P NMR of
the complex 1 contained a singlet at d ꢀ22.35 ppm.
The presence of a singlet strongly supported the idea
that the 31P nuclei are equivalent and indicated that
the PPh3 groups are trans disposed.
Complex 1 crystallizes in the monoclinic space group
P21/c [8]. It consists of a macro cationic [Os(j3-
tptz)(PPh3)2Cl]+ part, its neutrality is achieved by
[BF4]ꢀ and a water molecule is present in the crystal lat-
tice. The osmium core is in a +2 oxidation state and is
octahedrally coordinated, with three coordination sites
occupied with the tptz ligand [Os–N(1) 2.105(3); Os–
˚
N(2) 1.935(3); Os–N(3) 2.092(3) A], two for the trans-
PPh3 groups [P(1)–Os–P(2) 178.96(4); Os–P(1)
˚
2.4097(11); Os–P(2) 2.4220(11) A] and one for the
˚
chloro group [Os–Cl 2.4559(11) A] which is trans to
the nitrogen atom of the central triazine ring [N(2)–
Os–Cl 177.08(10)] (Fig. 1) [9]. An interesting right and
left-handed helical strand has been revealed in the crys-
tal structure of the complex 1. Crystal packing of the
complex shows an extended helical superstructure (Fig.
2), where the stacked association of helical domains with
opposite direction of growth and different arrangements
of the metal complex, (Fig. 3(a)) by a series of two
Fig. 3. (a) Opposite configuration of the metal complex unit for the complex 1 resulted via various intermolecular interactions. (b) Various
interactions C–H. . .X (X = F and p) involved in the stabilization of helical recimate for the complex 1.

3108
S.K. Singh et al. / Journal of Organometallic Chemistry 690 (2005) 3105–3110
equivalent pairs of intermolecular interaction generates
a racemate of helical architecture [9c]. The two strands
alternates up and down. One potential driving force
for such stacking (Fig. 3a & b) is the various C–
˚
Hꢁ ꢁ ꢁp(2.807 A) interactions involving phenyl rings of
PPh3 and C–Hꢁ ꢁ ꢁF (of BFꢀ4 ) interactions [C3–H3ꢁ ꢁ ꢁF3
˚
2.560; C11–H11ꢁ ꢁ ꢁF2 2.548 A] involving the pyridyl ring
of the ligand and the counter anion, which exists be-
tween adjacent strands (Table 2) [10]. It is also evident
that the water molecule present in the crystal lattice
plays an important role in the stabilization of these heli-
cal superstructures by acting as a linker between the
˚
complex units [O–Hꢁ ꢁ ꢁN6 2.211; O–Hꢁ ꢁ ꢁF4 2.326 A].
It is noteworthy that due to the lack of void space in
these densely packed complex molecules, the water mol-
ecules prefers to act as linker instead of occupying the
voids [11]. The independent and anti-parallel behavior
of these strands can be correlated with the opposite
arrangement of the linkers (water molecule and counter
anion BFꢀ4 ) as well as the metal complex in both the
strands. The pitch of the helix is calculated to be
Fig. 4. Absorption spectra of [Os(j3-tptz)Cl(PPh3)2]+ (50 lM), in
buffer A in the presence of increasing amount of CT DNA (0–100 lM).
Inset represents plot of (ea–ef)/(ea–ef) vs. [DNA]lM for [1]+. The best-fit
data for the absorption titration [Kb = 6.6 · 103 Mꢀ1 (s = 0.12)].
˚
24.915(8) A containing four metal centers (two of each
Table 2
Relevant weak interaction parameters for complex 1
strand) per turn for both right- and left-handed strands
[12]. The individual complex unit possibly in a C2-sym-
metric state (with few distortion), loses this state upon
the generation of such huge architecture and gains an
unsymmetric C1 state for the recemate.
˚
˚
X–Hꢁ ꢁ ꢁY
d Hꢁ ꢁ ꢁY (A) D X–Hꢁ ꢁ ꢁY (A) h X–Hꢁ ꢁ ꢁY (ꢁ)
O(1)–H(101)ꢁ ꢁ ꢁN(6) 2.21
O(1)–H(102)ꢁ ꢁ ꢁF(4) 2.33
C(11)–H(11)ꢁ ꢁ ꢁF(2) 2.55
C(20)–H(20)ꢁ ꢁ ꢁCl(1) 2.80
C(32)–H(32)ꢁ ꢁ ꢁCl(1) 2.70
C(44)–H(44)ꢁ ꢁ ꢁCl(1) 2.71
C(54)–H(54)ꢁ ꢁ ꢁCl(1) 2.68
2.9953
3.0097
3.4587
3.6527
3.4003
3.5544
3.5263
156
155
166
153
133
152
151
Extensive studies have been conducted to exploit the
mode of interaction of metal complexes with DNA [13].
It has been shown that both intercalation and surface
binding are the most acceptable binding modes for
non-planar octahedral complexes. Another non-classical
mode is partial intercalation where the planar chromo-
phore does not intercalate fully. In such cases the role
of ancillary ligands becomes prominent, as these may
prevent deep intercalation between the base pairs due
to steric hindrance [13c]. In the present case, we have
followed DNA interaction studies by an absorption
titration method [5]. Buffer A (5 mM Tris–HCl, pH
7.1, 50 mM NaCl) was used for absorption titration.
Fig. 4 depicts the absorption isotherm obtained for com-
plex 1 with calf thymus DNA. Data were obtained after
equilibration of the absorption titration of a fixed
amount of the complex 1 (50 lM) with increasing
amounts of calf thymus DNA (0–100 lM) at pH 7.1.
The binding constant (ꢀKbꢁ) and base pair sites (ꢀsꢁ) for
complex 1 with calf thymus DNA were analyzed quanti-
tatively by noting its decay in absorbance with increas-
ing concentration of calf thymus DNA, following the
model of Bard and Thorp for non-cooperative and
non-specific binding [14]. The calculated value of Kb
and s for complex 1 are 6.6 · 103 Mꢀ1 and 0.12, respec-
tively. (s < 1 is comparable with other reports, s = 0.02)
[13b,15]. The observed value of Kb and s for the osmium
complex is lower than that of observed for the analo-
gous ruthenium complexes, Os(tptz) < Ru(tptz) [5b].
The extracted binding constant for this complex shows
moderate binding with calf thymus DNA and this may
be due to steric hindrance caused by bulky phosphines,
which block further intercalation of the planar part of
the tptz ligand between the base pairs. The absorption
spectra exhibited 65% hypochromism and a bathochro-
mic shift of 5 nm suggesting partial intercalation of the
planar part of tptz ligand in the complex 1 is probably
the mode of interaction [5b]. Further, the presence of
an isobestic point at 278 nm strengthened the above fact.
3. Conclusions
We have shown that the Os(II) polypyridyl complexes
[Os(j3-tptz)(EPh3)2Cl]+ presents an excellent example of
an three-dimensional recemate of right and left-handed
anti-parallel stacked helical strands, where such a self
assembled process was assisted by counter ions and
the water molecule of crystal lattice. We have also de-
tailed on interaction study with calf thymus DNA and
found a weak interaction. More detailed investigations
towards reactivity of the complexes leading to substitu-

S.K. Singh et al. / Journal of Organometallic Chemistry 690 (2005) 3105–3110
3109
(b) S. Sharma, M. Chandra, D.S. Pandey, Eur. J. Inorg. Chem.
(2004) 3555;
tion products, biocatalytic aspects, and self-assembling
processes are in progress.
(c) S. Sharma, S.K. Singh, M. Chandra, D.S. Pandey, J. Inorg.
Biochem. 99 (2005) 458.
[6] (a) M.H.V. Huynh, D.G. Lee, P.S. White, T.J. Meyer, Inorg.
Chem. 40 (2001) 3842;
Acknowledgments
(b) P. Paul, B. Tyagi, A.K. Bilakhiya, P. Dastidar, E. Suresh,
Inorg. Chem. 39 (2000) 14;
We are grateful to CSIR, New Delhi, India for finan-
cial support [HRDG, 01(1784)/02/EMR-II]. Thanks are
also due to the Head, SAIF, Central Drug Research
Institute, Lucknow, for analytical and spectral facilities,
Prof. P. Mathur, Department of Chemistry, IIT Bom-
bay, for encouragement and National Single Crystal
X-ray Diffraction Facility, IIT, Bombay for providing
single crystal X-ray data.
(c) P. Paul, Proc. Indian Acad. Sci. (Chem. Sci.) 114 (2002)
269.
[7] (a) Synthesis of [Os(j3-tptz)(PPh3)2Cl]BF4 Æ H2O (1): A methan-
olic solution of ammonium hexachloro osmate(II) (0.430 g,
1.0 mmol) was added to a refluxing solution of triphenylphosphine
(1.578 g, 6.0 mmol) in methanol (50 mL) and the resulting
solution was refluxed for 1 h. Tptz (0.312 g, 1.0 mmol) was added
to the resulting suspension and the contents of the flask were again
refluxed for 8–10 h. The dark purple solution was cooled at room
temperature and was filtered to remove any solid residue. The
filtrate was reduced to one fourth of its volume under pressure. A
saturated solution of ammonium tetrafluoroborate dissolve in
methanol was added to the resulting concentrated solution and
left for slow crystallization in the refrigerator at ꢂ4 ꢁC. A
microcrystalline product was obtained and which was separated
by filtration and washed repeatedly with methanol (2 · 10 mL)
and diethyl ether (2 · 10 mL) and dried under vacuum. X-ray
suitable block shaped crystals were grown by diffusion technique
from dichloromethane and pet ether (60–80 ꢁC) Yield: 0.874 g
(76%). Anal. Calc. for BC54ClF4H44N6OP2Os: C, 55.48; H, 3.77;
N, 7.19. Found: C, 55.64; H, 3.69; N, 7.24. m/z (observed, relative
intensity, assignments): 1063, 65, [M + 1]; 800, 80,
[M ꢀ PPh3 + 1]; 764, 48, [M ꢀ PPh3 ꢀ Cl + 1]; 500, 10,
[M ꢀ PPh3 ꢀ Cl ꢀ PPh3 + 1]. 1H NMR (d ppm, CDCl3): 9.59 (d,
2H, J = 5.4), 9.18 (d, 1H, J = 7.5), 9.08 (d, 1H, J = 7.5), 8.80 (t,
1H, J = 7.8), 8.66 (d, 2H, J = 7.8), 8.19 (t, 1H, J = 6.0), 8.09 (t,
2H, J = 7.5), 7.86 (t, 1H, J = 6.3) and 7.29–7.17 ppm (br.m, 15H,
aromatic protons of PPh3). 31P NMR (d ppm, CDCl3): ꢀ22.35 (s).
UV–Vis {CH2Cl2, kmax nm (e Mꢀ1 cmꢀ1)}: 476 (6.1 · 103), 346
(5.7 · 103), 290 (1.4 · 104), 252 (2.3 · 104);
Appendix A. Supplementary data
1H–1H COSY spectra, Tables including bond param-
eters and full scale UV-Visible titration spectra for
complex 1 are available as supporting materials. Crystal-
lographic data for the complex 1 have been deposited
with the Cambridge Crystallographic Data Centre,
CCDC reference number 245835 in CIF format. Copies
of this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: +44 1223 336033; e-mail: deposit@ccdc.
cam.uk or www://ccdc.cam.ac.uk). Supplementary data
associated with this article can be found, in the online ver-
sion, at doi:10.1016/j.jorganchem.2005.03.051.
(b) Synthesis of [Synthesis of [Os(j3-tptz)(AsPh3)2Cl]BF4 (2): It
was prepared following the above procedure, except using
triphenylarsine (1.830 g, 6.0 mmol) in place of triphenylphos-
phine. Yield: 0.913 g (74%). Anal. Calc. for BC54ClF4H42N6A-
s2Os: C, 56.25; H, 3.82; N, 7.29. Found: C, 56.42; H, 3.67; N, 7.32.
m/z (observed, relative intensity, assignments): 1047, 59 [M + 1],
842, 64 [M ꢀ AsPh3 + 1], 806, 35, [M ꢀ AsPh3 ꢀ Cl + 1]. 1H
NMR (d ppm, CDCl3): 9.62 (d, 2H, J = 5.4), 9.21 (d, 1H,
J = 7.6), 9.04 (d, 1H, J = 7.5), 8.96 (t, 1H, J = 7.7), 8.72 (d, 2H,
J = 7.8), 8.27 (t, 1H, J = 6.2), 8.12 (t, 2H, J = 7.5), 7.95 (t, 2H,
J = 6.2) and 7.30–7.2 (br.m, 15H, aromatic protons of AsPh3).
UV–Vis {CH2Cl2, kmax nm (e, Mꢀ1 cmꢀ1)}: 481 (5.8 · 103),
351(sh), 292(sh), 257(sh).
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